1. Field of the Invention
This invention relates to a terminating interface of a control channel with a large degree of freedom at a digital multiplexing interface. It also relates to a testing device for performing tests of sending and receiving signals.
2. Description of the Related Art
Conventionally, an interface with a transmission speed of 1.544 Mbits/sec on which 24 digital channels are multiplexed and an interface with a transmission speed 2.048 Mbits/sec on which 32 digital channels are multiplexed have been adopted as a digital multiplex interface.
At such an interface, an individual line signal method of sending and receiving control signals using an individual communication line has been adopted as a method of sending and receiving control signals among stations, such as one containing call control information.
However, as various services are offered when an ISDN (Integrated Services Digital Network) is being put into service, both the quantity and variety of interstation control signals are increased, and a signal method with a higher speed and a larger capacity is demanded. Therefore, existing individual line signal methods are inadequate for a sending and receiving interstation control signals.
Thus, recently, a common line signal method of sending and receiving a control signal with a common data line has been adopted. A No. 7 common line signal method and a D-channel common line signal method are representative common line signal methods for ISDN. The former is a signal method fit originally for a voice communication, and the latter is a method fit for a data communication. Because these two methods differ only in their data formats and are similar in their basic transmission methods, the following explanation is made for the No. 7 common line signal method.
FIG. 1 shows the configuration of a commonly used digital signal network. Each PBX (Private Branch Exchange) 101 that accommodates a user terminal is connected to the public network P via a transmission device 102 from a digital multiplex interface 103. Examples of digital multiplex interface 103 include one called "30B+D", whose transmission speed is 2.048 Mbits/sec and comprises 30 B channels and a D channel (hereafter called 2M multiplex interface), and a digital multiplex interface called "23B+D" whose transmission speed is 1.544 Mbits/sec and comprises, 23 B channels and a D channel (hereafter called 1.5M multiplex interface).
FIGS. 2A and 2B respectively show the data formats of the above described 2M multiplexing interface and the 1.5M multiplexing interface. In the 2M multiplexing interface shown in FIG. 2A, channel C for a frame control is assigned to channel number (time slot) 00 on a transmission route, and channel D for transmitting a control signal is assigned to channel number 16. 30 channels of line channel B for user information transmission are assigned to other channels, namely, channels 01 through 15 and channels 17 through 31. In the 1.5M multiplexing interface shown in FIG. 2B, bit F for a frame synchronization is assigned to the forefront bit on the transmission line, and channel D for transmitting a control signal is assigned to channel number 23. 23 channels of line channel B for user information transmission are assigned to other channels, i.e. channels 01 through 22.
Here, as shown in FIG. 1, when a network is configured by using a public network p like an ISDN exclusively for a digital signal, an efficient control signal transmission is made available by using D channel, whose position is predetermined on a fixed basis.
However, when a control signal is sent and received by configuring a digital multiplexing interface using an existing network (for instance for a certain intracorporate communications), the method of using the D channel whose position is fixed, as in FIGS. 2A and 2B, has disadvantages. For example, when a total of five existing digital multiplexing transmission routes comprising three transmission routes of 5B+D (five B channels and a D channel; similarly abbreviated below), a transmission route of 4B+D, and a transmission route of 7B+D, is terminated and connected to a PBX with a 2M multiplex, the B channel has only to terminate 26 channels. Meanwhile, in this case, the D channel has to terminate 5 channels. Yet, the 2M multiplexing interface defined so far has a fixed form of 30B+D as shown in FIG. 2A and cannot meet the demand described above "as is".
As discussed above, in an intra-corporate network that does not need a public network, an existing digital multiplexing transmission route is often terminated. Accordingly, a channel that handles a control signal must be flexibly assigned to the terminated digital multiplexing interface. However, an existing digital multiplexing interface where the number and position of the D channel are fixed has a problem of being incapable of efficient termination of the control channel in the intracorporate network discussed above.
On the other hand, it is crucial for stable operation of the network to test the sending and receiving of a signal at each terminating device at the beginning or during the course of the network operation. Needless to say, such a test of sending and receiving a signal is necessary for an intracorporate network having a digital multiplexing interface, as discussed above.
FIG. 3 shows a commonly used method of testing sending and receiving signals. When a PBX 301, which is a terminating equipment of the home station A, is connected to a digital multiplexing transmission route (digital transmission line) via a transmitter 302, the following test of sending and receiving signals are usually made. The sending and receiving voice is tested by having a telephone device 303, terminated in the home station A, call up another telephone device 303 terminated in the counter station B. The sending and receiving data is tested by having a data terminal 304, terminated in the home station A, call up another data terminal 304 terminated in the counter station B.
However, this testing method has a problem of always requiring a counter station B or a device that realizes the same function as the counter station. Besides, the above testing method has the problem of having to simultaneously work on the home station A and the counter station B, even if the counter station B can indeed be prepared. Furthermore, in the above testing method, the digital multiplex transmission route, which is an actual line for connecting the home station A and the counter station B, must be in a usable condition. But in reality, there is a problem that the digital multiplex transmission route is usable at the time of testing.